The present invention relates to an improved method for the recovery of oil from subterranean hydrocarbon bearing formations containing low API gravity viscous oils or bitumens. More particularly, the invention relates to the production of bitumens and hydrocarbons from reservoirs of low mobility such as tar sand formations.
The recovery of viscous oils from formations and bitumens from tar sands has generally been difficult. Although some improvement has been realized in stimulating recovery of heavy oils, i.e., oils having an API gravity in the range of 10.degree. to 25.degree. API, little, if any, success has been realized in recovering bitumens from tar sands. Bitumens can be regarded as highly viscous oils having a gravity in the range of about 4.degree. to 10.degree. API and contained in an essentially unconsolidated sand referred to as tar sands.
Vast quantities of tar sands are known to exist in the Athabasca region of Alberta, Canada. While these deposits are estimated to contain several billion barrels of oil or bitumen, recovery therefrom using conventional in situ techniques has not been too successful. The reason for the lack of success relate principally to the fact that the bitumen is extremely viscous at the temperature of the formation, with consequent low mobility. In addition, these tar sand formations have very low permeability, despite the fact they are unconsolidated.
Since it is known that the viscosity of oil decreases markedly with an increase in temperature, thereby improving its mobility, thermal recovery techniques have been investigated for recovery of bitumens from tar sands. These thermal recovery methods generally include steam injection, hot water injection and in situ combustion.
Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation. In a steam operation employing two wells, steam is introduced into the formation through the injection well. Upon entering the formation, the heat transferred by the hot fluid functions to lower the viscosity of oil, thereby improving its mobility, while the flow of the hot fluid functions to drive the oil toward the production well from which it is produced.
In the conventional forward in situ combustion operation, an oxygen-containing gas, such as air, is introduced into the formation via a well, and combustion of the in-place crude adjacent the well bore is initiated by one of many known means, such as the use of a downhole gas-fired heater or a downhole electric heater or chemical means. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain a combustion front which is formed, and to drive the front through the formation toward the production well.
As the combustion front advances through the formation, a swept area consisting, ideally, of a clean sand matrix, is created behind the front. Ahead of the advancing front various contiguous zones are built up that also are displaced ahead of the combustion front. These zones may be envisioned as a distillation and cracking zone, a condensation and vaporization zone, an oil bank and a virgin, or unalterated zone.
The temperature of the combustion front is generally in the range of 650.degree.-1200.degree.F. The heat generated in this zone is transferred to the distillation and cracking zone ahead of the combustion front where the crude undergoes distillation and cracking. In this zone, a sharp thermal gradient exists wherein the temperature drops from the temperature of the combustion front to about 300.degree.-450.degree.F. As the front progresses and the temperature in the formation rises, the heavier molecular weight hydrocarbons of the oil become carbonized. These coke-like materials are deposited on the matrix and are the potential fuel to sustain the progressive in-situ combustion zone.
Ahead of the distillation and cracking zone is a condensation and vaporization zone. This zone is a thermal plateau and its temperature is in the range of from about 200.degree. to about 450.degree.F., depending upon the distillation characteristics of the fluid therein and formation pressure. These fluids consist of water and steam and hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank which forms as the in-situ combustion front progresses and the formation crude is displaced toward the production well. This zone of high oil saturation contains not only reservoir fluids, but also condensate, cracked hydrocarbons and gaseous products of combustion which eventually reach the production well from which they are produced.
Various improvements relating to in-situ combustion are described in the prior art that relate to the injection of water, either simultaneously or intermittently with the oxygen-containing gas to scavenge the residual heat in the formation behind the combustion front, thereby increasing recovery of oil. Prior art also discloses regulating the amount of water injected so as to improve conformance or sweep and to control the cumbustion.
Experience has generally shown that these conventional thermal techniques have not been altogether successful when applied to the recovery of heavy oils or bitumen. Where the hydrocarbons sought to be produced have a low API gravity, the build-up of the oil bank ahead of the thermal front occurs to a great extent. Since the heat transfer is low ahead of the front, these heavy hydrocarbons become cool and hence immobile, thereby causing plugging of the formation with the result that the injection of either air in the case of in-situ combustion, or steam in the case of steam is greatly restricted.
Furthermore, in the case of in-situ combustion, when applied to heavy oils the high molecular weight fractions are carbonized. These carbonaceous deposits serve as the fuel for the in-situ combustion reaction, but because the oil contains such a high percentage of these fractions, very high fuel requirements are incurred with consequent low recovery and high oxygen requirements.
The difficulties recited above become compounded when these techniques are applied to the tar sands, because not only do the bitumens have a low API gravity, i.e., 6-8.degree. API and a higher viscosity, i.e., in the millions of centipoises, but also the permeability of the tar sands is so low that difficulty has been experienced in establishing fluid communication within the formation.
Accordingly, it is an object of the present invention to provide an improved recovery method whereby both highly viscous low gravity crudes and bitumens can be recovered more efficiently. The instant invention accomplishes this by a combination of in-situ low temperature oxidation and hot water or steam injection into the upper or lower portion of the formation whereby the oxidation is controlled by the selecting of the ratio of the oxygen in the oxygen-containing gas to the water that is injected either as hot water or steam.